In elevator systems, elevator car doors are driven into opened and closed positions by an electric motor. The elevator car doors usually couple to hoistway doors to drive hoistway doors into open and closed positions. The elevator car doors typically have to maintain a certain velocity at the beginning of an operation, in the middle of the operation, and at the end of the operation. For example, as the doors are opening, the initial velocity is relatively low to allow time for the elevator car doors to couple to the hoistway doors. After the two sets of doors are coupled, the doors accelerate to a higher velocity. The doors then decelerate toward the end of the opening operation to avoid slamming against a fixed stop.
A number of methods are used to achieve the change in velocity of the elevator car doors during various operations. A conventional approach to changing the velocity of elevator car doors is to use resistors. The resistors are placed in series with a voltage source and a DC motor and adjusted to provide smaller or greater values of resistance. The greater resistance value corresponds to slower DC motor operation and to smaller generated velocity output. The converse is true for the smaller resistance values. However, this approach has a number of limitations. First, resistors cannot compensate for changes in friction or other loading on the doors. Second, when resistors heat up, the resistance value changes and results in changes in the velocity of the elevator car doors. Such changes in velocity are highly undesirable because this does not provide a consistently smooth profile. Third, resistors are adjusted by a trial and error method, which is time consuming and frequently lacks the necessary precision.
Another approach to varying the velocity of elevator car doors in modern closed loop systems includes software generated velocity profiles. The software dictates what the door velocity should be at a given time or distance. The velocity profiles are generated for each operation of the elevator car doors. This approach either results in a time lag for the doors to respond to a command or requires a powerful processor to generate profiles in real time.
Additionally, it is highly desirable to have smooth transitions from one velocity value to another. The transition from one velocity level to another is currently achieved by building in constant jerk (rate of change in acceleration divided by rate of change in time) segments and constant acceleration segments of the profile. The constant jerk segments, used to smooth the corners of the transition from constant velocity phase to constant acceleration phase of the doors, must match values of velocity and acceleration where the constant jerk segments join the constant acceleration phase and the constant velocity phase. Matching the constant jerk segment with the constant acceleration phase and the constant velocity phase at numerous transition points takes a great deal of processor time.
Furthermore, a door control system cannot cause the doors to follow the high frequency components of the profile if these frequency components are higher than the bandwidth of the control system. This can cause misoperation of the elevator car doors, such as the doors overshooting the final position and hitting the stops or exciting resonant vibration frequency of the doors. The velocity profiles can be broken down to show the frequency content through a Fourier transform method. The profiles include low and high frequency components of velocity. Although the current method of providing constant jerk segments lowers the frequency content of the velocity profile by minimizing sharp corners, it is not known how much of the higher frequency content is attenuated and what the frequency content of the profile is. Besides causing misoperation, the constant jerk method of generating a profile could result in very low frequency content and an associated increase in door operation time as compared to the optimum operation time.